The invention relates to electric circuit breaker contact arm assemblies similar to that described in U.S. patent application Ser. No. (479617) entitled "Contact Arm Assemblies For Use In Electric Circuit Breakers" and to the circuit breaker operating mechanism described in U.S. Pat. No. 3,605,052 to Dimond et al. Both the Application and the Patent are incorporated herein for purposes of reference.
Since molded case electric circuit breakers are used over a wide range of continuous current ampere ratings, it is desirable for purposes of manufacture to use a standard tripping and contact operating mechanism for a particular breaker line and to increase the size of the operating components in proportion to the current rating. The circuit breaker described within the Patent to Dimond et al., for example, is useful over a range of continuous available currents from 15-150 amperes. The size of the bi-metallic elements, contacts, contact arms and contact springs would vary depending upon the particular rating.
One of the difficulties encountered with the "scaling-up" of the components size is the size restriction limited by the dimensions of the circuit breaker casing. A further difficulty arises in the nature of the severity of the qualifying tests that the higher rating breakers are subjected to, particularly the short circuit current test requirement. After subjection to short circuit test, the breaker is retested to ensure that it is still electrically as well as mechanically operable. During short circuit tests, the breaker is subjected to extreme thermal and mechanical stresses which could excessively damage the operating components.
Another difficulty encountered in breaker "scaling-up" design is the interaction of the contact spring with the movable contact arm. The purpose of the contact spring is to ensure minimum electrical resistance between the contacts when the contacts are closed. This contact spring pressure must be overcome by the operating mechanism spring when called upon to close the contacts. Most contact springs are found to exhibit an increase in spring force wherein the force on the contact arm increases as the contact arm is subjected to magnetic repulsion forces under short circuit tests. As the contact arm moves the contact to its open position the retarding contact spring force actually increases. It would be desirable therefore to provide a contact spring such that the spring force rapidly decreases under short circuit conditions while the contacts are blown open under the intense magnetic repulsion forces. The aforementioned U.S. patent application describes a movable contact arm assembly wherein the contact spring exhibits a fairly constant spring force as the contact arm moves to its fully open position. The purpose of this invention is to provide a contact operating arm assembly wherein the force biasing the contacts in a closed position rapidly approaches zero as the contact arm moves to its fully open position.